Grid modulated amplifier



May 27, 19415 T. M. GLUYAS, JR

GRID MODULATED AMPLIFIER Filed. March 15, 1959 2 Sheets-Sheet 1 oqdowwo qo MM F Jauxcs May 27, 1941. 'r. M. GLUYAS, JR

GRID MODULATED AMPLIFIER Filed March 15', 1939 2 Sheets-Sheet 2 SvukcE' Patented May 27, 1941 2,243,504 cam MODULATED AMPLIFIER.

Thomas M. Gluyas, Jr., Kansas City, Mo.,,as-

signer, by mesne assignments, to Philco Radio and Television Corporation, Philadelphia, Pa., a

corporation of Delaware Application March 15, 1939, Serial N0. 262,040

13 Claims.

This invention relates to grid-modulated amplifiers, and more particularly to grid-modulated thermionic amplifiers capable of operation at very high frequencies such as are employed in television transmissions and the like.

Particularly at the higher frequencies severe limitations of performance are imposed upon grid-modulated amplifiers by reason of inherent interelectrode capacities. certain of these capacities combine to form relatively low input and output impedances, and, as is well known in the art, tend to lower the maximum high frequency operating limits of the amplifier, to decrease the obtainable side band widths, and, moreover, to increase the amount of power which must be supplied by the modulator stage in order to secure the desired degree of modulation.

Still other of these interelectrode capacities may tend toward an unstable amplifier performance, and may even introduce undesired or parasitic oscillations over a wide range of frequencies. This effect is, in general, due to the feedback of energy from the output of a given tube to its input by way of the above mentioned interelectrode capacities. Similarly in some instances there may occur an appreciable transfer of energy from the input of the tube to its output by way of the same capacities, tending to occasion other disadvantageous effects. It is known to overcome this undesired capacitive transfer of energy by the provision of additional capacities for the purpose of neutralizing the effects of the undesired capacities, recourse being had, as is well understood, to certain bridge circuits. Obviously no physical capacitances are neutralized out of existence. Quite to the contrary, the total capacity associated with the amplifier is increased, resulting in increased input and output capacities. notwithstanding the very real reduction in capacitive coupling between input and output circuits of the neutralized amplifier. In consequence of these facts, it may be observed that in prior modulated amplifiers, particularly those of the grid-modulated class, stability of operation has been hitherto achieved only at the expense of decreased modulation efiiciency, and of reduced band width and upper operating frequency limits.

I have found it possible, by means of a neutralized, grid-modulated amplifier constructed in accordance with this invention, to secure the desired stability of operation without the serious limitations which have been inherent in prior circuits. The grid-modulated amplifier of this invention is cathode driven, that is to say, the radio-, or carrier-frequency input or driving voltage is applied to the cathode rather than to the control grid, the latter electrode being substantially at ground potential for voltages of the car- At high frequencies rier frequency. The control grid is used, however, as the input electrode for the modulating signal, its impedance to ground at modulation frequencies being relatively high. As will be explained indetail hereinafter, the resulting reduction in magnitude of the requisite neutralizing capacity is so considerable that important reductions are effected in input and output capacities as compared with those obtaining in prior grid-modulated amplifiers. The advantages resulting therefrom have already been briefly disclosed.

Accordingly one of the objects of the present invention is the provision of a grid-modulated amplifier wherein the carrierv input voltage is introduced to the cathode rather than to the grid electrode of the amplifier tube employed.

Another object of the present invention is the provision of a novel gridmodulated amplifier capable of being neutralized with neutralizing condensers much smaller than those required in prior circuits.

Another object of this invention is the provision of a grid-modulated amplifier possessing input and output capacities ofa smaller. order than has heretofore been possible. j

A further object of the invention is-the provision of a cathode driven amplifier capable of operation at ultra-high frequencies.

Still another object of this inventionjis the provision of a wide-band grid-modulated amplifier suitable for service at high frequencies and capable of modulation by high definition wideband television signals as well as by audio frequency signals.

A further object of this invention is the provision of a grid-modulated amplifier possessing unusually low internal feedback capacities.

In the drawings: 7

Fig. 1 is a schematic representation of a gridmodulated amplifier constructed in accordance with the prior art;

Fig. 2 is a schematic representation of a neutralized wide-band, cathode-driven, grid-modulated amplifier designed in accordance with the principles of the present invention;

Fig. 3 is a schematic representation of agridmodulated amplifier similar to that illustrated in Fig. 2, but employing resonant lines in place of lumped resonant circuits of the preceding figure;

Fig. 4 is a schematic representation of a single tube cathode-driven, grid-modulated amplifier employing the principles of the invention; and

Fig. 5 is a modification of Fig. 2 in which the functions of cathode and anode circuits are interchanged.

Referring to Fig. 1, there is illustrated a conventional grid-modulated amplifier comprising two triodes l and 2 in push-pull relation. The amplifier is driven from a source of carrier frequency 5, the triode .grids being coupled thereto by means of a pair of resonant circuits 3 and 3'. The modulating signal from source M, amplified by suitable amplifying means 6, is supplied to the grids of the triodes I and 2, in phase, by means of a suitable coupling network |89, connection being made to the midpoint of the inductor included in the resonant circuit 3. The desired modulated carrier output may be derived from the resonant output circuit 4 by means of a suitable coupling means 4'. Conventional neutralizing condensers N1 and N2 are shown and, as is well understood by those skilled in the art, the capacity of these condensers will be substantially equal to that of the grid-to-plate interelectrode capacities designated Cgp in the broken line representations. It will be noted that the two capacities cgp and the two neutralizing condensers N1 and N2 form a series-parallel capacitanceshunt across the output circuit 4. In addition, the two cathode-to-plate interelectrode capacities Cop form a series-connected shunt across the output circuit 4. Accordingly the total shunt capacity across the output circuit 4, and chargeable to the neutralizing capacities (N1=N2=cgp), the cathode-to-plate, and grid-to-plate interelectrode capacities, may be represented by the formula Co= /2(Ccp+2Cgp) (1) The grid-to-cathode interelectrode capacity is of no importance in estimating the total output capacity of such a neutralized circuit, and has accordingly been omitted, since it would drop out of the final equation in any event.

Allusion has already been made to the fact that the amount of modulating power required to effect modulation in a carrier frequency amplifier is a function of the input capacity of the amplifier. This is obvious, since it is apparent that the higher the input capacity (1. e., the lower the input impedance) offered to the modulating signal, the greater must be the power supplied to provide the necessary modulating voltage. Hence it is desirable to provide as low an input capacity as possible, and this is particularly true in television systems where the video modulating signal may contain signal components of the order of 5 megacycles or more, to which even small capacities will offer a comparatively low impedance. Since wideband video frequency amplifiers are inherently of low gain, the provision of large amounts of modulating power over such a band will be seen to be costly.

In a grid-modulated amplifier as illustrated in Fig. l, the input capacity is due largely to the grid-to-plate capacities, the cathode-.to-grid capacities, the neutralizing capacities, and the capacity of the input circuit 3 to ground, The cathode-to-plate capacities, Cop contribute virtually nothing to the input capacity and may be ignored in the consideration. In fact, since the impedance of the resonant circuit 4 is negligible at modulation frequencies, the triode plates should be assumed to be at ground potential in computing the input capacity of the amplifier for these frequencies. Similarly the impedance of the circuit 3 is negligible to voltages of the modulating frequencies, whence in computing the input capacity at these frequencies the two triode grids may be considered as bonded directly together and shunted to ground through four interelectrode capacities Cgp (two) and Cgc (two) and two neutralizing capacities N1 and N2, all connected in parallel. Accordingly, the total shunt capacity Cm presented to the modulating signal, and chargeable to the neutralizing capacities (N1=N2=Cgp), the grid-to-cathode capacities Cgc, and the grid-toplate capacities Cgp, and the capacity Ct of the input circuit 3 to ground, may be represented by the formula This input capacity Cm may be considered as a lumped capacity in shunt with the output of the modulating signal amplifier, and is so shown in the drawings.

Expressions have been derived for the output capacity Co and for the input capacity (to modulating signal voltages) Cm for a grid modulated amplifier of conventional design. The improved grid-modulated amplifier of the invention will now be described, and similar expressions for input and'output capacities derived in order to demonstrate the important reductions made in these undesirable capacities.

Attention is now directed to Fig. 2, wherein there is schematically represented a cathodedriven grid-modulated amplifier designed in accordance with the present invention. Two triodes, l0 and H, are again shown in push-pull relation, their plates being coupled to a load circuit H) by means of a resonant circuit l3--l4. Obviously the capacitance l 3 may be any suitably selected capacitor, or at very high frequencies may consist solely of the output capacity of the amplifier and the distributed capacity of the inductor 14. The amplifier is cathode-driven from a source of carrier frequency 5, the cathodes being coupled to the carrier frequency source by any suitable device l2. The modulating signal from source M is supplied to the grids of'the triodes I0 and H, in phase, as described in connection with Fig. 1. In Fig, 2, however, the grids are energized only by the modulating signal, and are substantially at ground potential for voltages of the carrier frequency.

Interelectrode capacities Cgp, Cgc, and Ccp are shown as in the previous illustration. It is important, however, that in the cathode-driven amplifier of the invention, the output-to-input (or vice versa) feedback of energy through the vacuum tubes is by way of the relatively small interelectrode capacity Cop rather than through the much larger capacity Cgp as in the former case. It will be understood by those skilled in the art that the capacity Cop is considerably smaller than the capacity Cgp as a result of the greater physical separation of plate and cathode as compared to that of plate and grid, and that this difference is further increased by the shielding effect between cathode and plate exercised by the interposed grid electrode which is substantially at ground potential to voltages of the carrier frequency. Consequently the neutralizing capacities N3 and N4, provided to neutralize the electrostatic transfer of energy through the tubes, are much smaller than in the previously discussed circuit, and will be substantially of the same magnitude as the small interelectrode capacities Cop. As will presently be demonstrated this is one of the factors responsible for the decreased input and output capacities of my cathode-driven grid-modulated amplifier.

The neutralization of the circuit of the present invention substantially eliminates the capacitive transfer of energy through the tube in either direction. Normally the transfer of energy from output to input (from plate to cathode in Fig. 2)

is of chief importance tending, as it may, to'instability, unsymmetrical generation of side bands, and parasitic oscillations; however, where triodes having a relatively low mu are employed, the voltage on the input electrode may not be many times less than the voltage on the output electrode, in which event the voltage across the plate circuit may be unfavorably influenced as a result of electrostatic coupling to the input electrode, in which event the voltage across the ultra-high frequencies, where complete modulation of the carrier may be rendered impossible. Accordingly it is seen that the neutralizing of such an amplifier is of particular importance at very high frequencies.

An important factor contributing to decreased input capacity, for signals of the modulating frequencies, results from the fact that the capacity to ground of the carrier frequency input circuit is no longer in shunt with the modulating signal circuit. In fact in Fig. 2 the resonant circuit I2 is substantially at ground potential to voltages of the modulation frequencies, a condition which could not be tolerated in prior systems such as that shown in Fig. 1 wherein the modulating signal must be built up between this resonant circuit and ground.

Considering now the output capacity of the amplifier of Fig. 2, it will be seen that it is composed largely of a series-parallel combination of the two capacities Ccp and the two neutralizing capacities N3 and N4 (where N3=N4=Ccp shunted by the two capacities Cg'p in series. Accordingly the total shunt capacity across the output circuit l3 4, and chargeable to the neutralizing capacities and the cathode-to-plate and grid-to-plate interelectrode capacities, may be represented by the formula The grid-to-cathode interelectrode capacity has been omitted from the derivation of Equation 3 as was done in the case of the corresponding Equation 1.

The input capacity of the amplifier of Fig. 2 (to voltages of the modulating frequency impressed between the grids and ground) is due primarily to the grid-to-plate and grid-to-cathode interelectrode capacities. It will be observed that the cathode-to-plate interelectrode capaci ties Cop, as well as the neutralizing capacities N3 and N; do not form any part of this input capacity, since for modulating frequencies the cathodes and plates are substantially at ground potential. Accordingly the four interelectrode capacities Cgp (two) and Cgc (two) effectively extend between the triode grids and ground, whence the total grid input shunt capacity to modulating frequencies is given by the formula cm 2(Cgc+Cgp) (4) Comparing the output capacities of the two systems, that of the prior art as shown by Equation 1, and that of the invention as shown by the Equation 3 it will be noted that theequations are generally of the same form but show the important difference that the factor 2 is applied to Cgp in the former and to Ccp in the latter. Since Cgp may be many times larger than Cop, as explained hereinbefore possibly ten times as large), it will be obvious that Co will be considerably smaller in my cathodedriven grid-modulated amplifier than in a grid-modulated amplifier designed in accordance with the prior art. The smaller output capacity possessed by my amplifier permits its operation at much higher carrier frequencies, and permits the generation of side band frequencies ,of substantially increased bandwidth, band-width being an inverse function of tank circuit capacity, the latter including both the output capacity of the amplifier and the distributed capacity of the tank inductor.

Comparing now the input capacities (to voltages of the modulating frequencies) of the two systems, that of the prior art as shown by Equation 2, and that of the present invention as shown by Equation 4, it will be noted that the input capacity of my cathode-driven grid-modulated amplifier is smaller than that possessed by an amplifier such as shown in Fig. 1 by a difference of 2cgp+ct. The elimination of this undesired shunting capacity as made possible by my invention contributes greatly to the simplification of design of the modulation frequency channel, particularly where a wide band of modulating frequencies is involved. Referring, for example, to Fig. 1 it will be understood that, in order to obtain substantially uniform modulating voltages across Cm over a wide band of frequencies, the shunt combination of the coupling resistors I and 8 must be of a relatively low impedance, comparable to the impedance of Cm at the upper limit of the modulation frequency band. This usually requires that a large number of space discharge devices 6 connected in parallel be employed to provide the high modulating power requiredthus further adding to the capacity in shunt with Cm and further complicating the problem.

In order more concretely to indicate the general order of reduction in both input and output capacity which can be efiected through the practice of this invention, there are herewith pr sented average interelectrode capacities obtaining in one type of air-cooled power amplifier triode suitable for use in the circuits of Figs. 1 and 2. Cgp=4:,lL/.Lf$. Ccg=3lLlLfS. ccp OfifL/Lf. The distributed capacity, Ct, of the tank circuit 3 to ground might be of the order of 12 ,u LfS. Substituting these values in the Equations 1 to 4 the following comparisons are obtained.

The output capacities Co yielded by Equations 1 and 3 are 4.3 ,lLlLfS. and 2.6 fs, respectively, these being the output capacities of the circuits of Fig. 1 and Fig. 2 respectively. Accordingly it is clearly apparent that my cathode-driven gridmodulated amplifier posessess an output capacity very substantially less than that obtaining in circuits representative of the prior art. More specifically, for the particular values cited, the reduction in capacity amounts to approximately 40% of that formerly obtaining.

Employing the same average values, the input capacities yielded by Equations 2 and 4 are 34 fs. and 14 is. for the circuits of Fig. 1 and Fig. 2 respectively. Here the results are even more highly in favor of the circuit of the invention, the input capacity being 59% less than that obtaining in circuits of the prior art.

These comparative figures are obviously only illustrative. Depending upon the equipment employed and upon the relative values of the interelectrode capacities possessed by a specific vacuum tube, the resulting reductions in input and output capacities may be more or less than those obtaining in the above example.

In addition tothe above described advantages of my grid-modulated amplifier, an important advantage lies in the fact that the capacity to ground of the carrier frequency input circuit does not enter into the determination of the input capacity associated with the modulating frequency channel, as it does in the circuits of the prior art. This is demonstrated by the fact that Ct appears in Equation 2, but does not appear in Equation 4. Accordingly far more latitude of design is possible in the construction of the carrier frequency input coupling elements. Moreover, in circuits such as that shown in Fig. 1 it has been general to use relatively loose coupling between the circuits 3 and 3 in order to keep the capacity to ground of the resonant tank 3 at a minimum. Where this type of input circuit is employed in the circuit of Fig. 2, a much tighter coupling is possible, resulting in improved radio frequency load regulation at this point.

Referring to Fig. 2 it may be noted that if the driving circuit 12 offers an appreciable impedance (i. e. between taps l6 and H) to currents of the carrier frequency, the amplifier Will tend to be degenerative. This, of course, results from the fact that a portion of the inductor associated with the resonant circuit l2 lies in the path of the output current of the amplifier. In some cases a highly degenerative system may be desired, but preferably the impedance of the portion of the driving circuit included in the path of the output current should be small. The impedance of the driving circuit may be made to appear small to the carrier frequency output currents by tapping down on the input inductor, as is done in the case of the tapped points It and I! in Fig. 2.

Reference may now be had to the circuit of Fig. 3 which illustrates the system of Fig. 2 when resonant lines 20 and 2| are substituted for the lumped resonant circuits I 2 and l3|4 respectively. Neutralizing condensers N5 and N6 are again employed to neutralize the efiects of capacitive energy transfer within the tubes. Since the principles involved in this system are similar to those already discussed in connection with Fig. 2, the various interelectrode capacities have been omitted from the drawings. Similarly the source of modulating signal voltage has merely been indicated generally by the letter M; it may comprise any suitable modulating equipment capable of impressing the desired signals on the grids of the triodes l8 and I9. A suitable load circuit may be coupled to the resonant line 2| by means of the taps 22 and 23. resonant lines themselves may be of the quarterwave type, and may be short-circuited at one end, as indicated in the figure. The grids are again substantially at ground potential for voltages of the carrier frequency since the impedance of the capacity Cm in parallel with the shunt capacity associated with the modulating voltage source M will be relatively small at the carrier frequency. The circuit of Fig. 3 is particularly desirable where a very high carrier frequency is employed and where quarter-Wave lines are of practical physical dimensions.

While the specific embodiments illustrated in Figs. 2 and 3 employ a push-pull connection of triodes it will be obvious that the invention is fully adapted for application to single tube circuits. Such an adaptation is illustrated in Fig. 4 wherein a single triode 24 is cathode-driven from a source of carrier frequency voltage 5. Coupling between the source and the cathode may be through any suitable means such as a resonant circuit 26 tuned to the carrier frequency. A suitable modulating voltage M may be applied to the grid of the triode as shown. A load,

The

such as an antenna, may be supplied from the terminals of the inductor 21, coupled into the anode circuit of the triode by means of a resonant circuit 25. In the illustration, the triode plate is supplied with plate voltage by means of .a tap 3+ on the inductor associated with the resonant circuit 25. Accordingly the lower end of the resonant circuit presents a source of voltage Which is substantially out of phase with that on the plate of the triode. The neutralizing condenser N7 may then be connected fromthis point on the resonant circuit to the cathode or to some other suitable point on the carrier frequency input circuit. There are, of course, other known means for obtaining the desired out-ofphase voltage for the neutralization of radio frequency amplifiers, and the invention is by no means to be limited to the specific method illustrated. Since the circuit of Fig. 4 can be derived from Fig. 2 merely by the removal of triode II and the neutralizing condenser N4, and by a slight modification of the carrier frequency input circuit l2 it is believed that further explanation is unnecessary.

Following the more usual practice, the anode or plate electrodes of the vacuum tubes have been selected as the output elements in the foregoing illustrations. While this selection is normally preferred, it should :be clearly understood that the functions of plate and cathode may be interchanged. Thus, the cathode may be employed the output electrode, while the plate may be used as the carrier input electrode. With tubes possessing a low mu, the latter mode of operation may differ very little from the more usual mode. The embodiments of Figs. 2, 8 and 4 might be adapted to the plate-driven mode (with cathode output), by interchanging the carrier sources and load circuits. Thus, in Fig. 4 the R. F. source 5 might be coupled to the resonant plate circuit 25, while the load circuit 21 might be coupled to the tuned cathode circuit 26. Obviously it might be necessary to make certain routine circuit changes to secure the desired impedance matches, load conditions, bias voltages, and the like. It may 'be noted that the representative prior art circuit of Fig. 1 is not adapted for such reversible operation.

Fig. 5 is representative of such reversed operation. Referring to this figure it will be noted that the anodes and cathodes of the triodes 29 and 30 are connected in push-pull relation by the resonant circuits 28 and 3| respectively. This amplifier circuit differs from the preceding ones in that the resonant cathode circuit 3| is employed as an output circuit being coupled to a subsequent amplifier or to an antenna by means of the coupling winding 32, while the resonant anode circuit 28 is employed as the input win-ding, being supplied with an R. F. driving voltage from the source 5. Neutralizing condensers Na and N9 may again be provided as they were in Figs. 2 and .3. The modulating voltage source M may, of course, include any bias voltage necessary to permit the amplifiers 29 and 30 to operate over a desired portion of their characteristics. Such a bias may be provided, for example, as indicated in Fig. 2.

For the reasons above stated, the terms "input electrode and output electrode, as employed in the claims, are to be construed broadly with reference to the cathode and anode.

While the illustrated embodiments of the invention show the use of neutralizing condensers N3 to N7, it is pointed out that these condensers are not inherently necessary to the practice of this invention which relates broadly to a cathodedriven grid-modulated amplifier. Thus, at low carrier frequencies the neutralizing condensers may be omitted, resulting in an even greater reduction in the output capacity, as will be apparent from the derivation of Equation 3. On the other hand, when high carrier frequencies are employed the maximum benefits of the invention may be realized by the use of small neutralizing condensers, as hereinbefore explained.

Although the invention has been described with particular reference to the embodiments of the drawings, it will be understood that the invention is capable of various forms of physical expression. For example, while triodes have been illustrated in the several drawings it should be evident that multi-electrode tubes might also be advantageously employed. The invention is, therefore, not to be limited to the specific disclosure, but only by the scope of the appended claims.

I claim:

1. Apparatus for producing a carrier signal modulated with an intelligence signal, comprising a space discharge tube having a cathode, a grid, and an anode, a source of driving voltage of carrier frequency, means for connecting said source between saidcathode and a point of predetermined potential, an output load circuit associated with said anode, capacitive means external to said discharge tube for preventing the capacitive transfer of energy within said tube, a source of modulating intelligence signal, means for connecting said intelligence signal source between said grid and said point of predetermined potential, and a capacitive path having low impedance to currents-of said carrier frequency connected between said grid and said point.

2. Apparatus for producing a carrier signal modulated with an intelligence signal, comprising a space discharge tube having a cathode, a grid, and an anode, a source of driving voltage of carrier frequency, means for connecting said source between said cathode, and a point of predetermined potential, said grid being substantially at ground potential with respect to said driving voltage, an output load circuit associated with said anode, a neutralizing condenser connected between said cathode and a circuit point at which there is a carrier frequency voltage of substantially opposite phase to that of the carrier voltage on said anode, a source of modulating intelligence signal, means for connecting said intelligence signal source between said grid and said point of predetermined potential, and an electrical path having negligible impedance at said carrier frequency connected between said grid and said point of predetermined potential, said grid serving as an electrostatic shield between said anode and said cathode.

3. Apparatus for producing a carrier signal modulated with an intelligence signal, comprising a vacuum tube having at least a cathode, grid, and anode, a source of driving voltage of carrier frequency, means for connecting said driving voltage between said cathode and a point at ground potential, means for maintaining said grid at substantially ground potential for voltages of said carrier frequency, a resonant circuit connected to said anode and tuned to said carrier frequency, a neutralizing condenser connected between said cathode and a point on said resonant circuit, the voltage of said last-named point being of substantially opposite phase to that on said anode, a source of modulating intelligence sign-a1 having a frequency substantially less than said carrier frequency, and means for connecting said intelligence signal source between said grid and a point of ground potential.

4. Apparatus for producing a carrier signal modulated with an intelligence signal, comprising two space discharge devices each having a cathode, grid, and anode, an output load circuit connecting the anodes of said devices in push-pull relation, a source of driving voltage of carrier frequency, a carrier frequency input circuit connecting said cathodes in push-pull relation, means for coupling said source to said input circuit, a neutralizing condenser connected between the cathode of one of said devices and a point Whose potential is of substantially opposite phase to the potential on the anode of said one device, a second neutralizing condenser connected between the cathode of the other of said devices and a point whose potential is of substantially opposite phase .to the potential on the anode of said other device, a source of modulating intelligence signal, a direct low-impedance connection between the grids of said space discharge devices, means for connecting said modulating intelligence signal source between said grids and a point of predetermined potential, and means for maintaining said grids substantially at ground potential with respect to said carrier frequency voltage.

5. Apparatus for producing a carrier signal modulated with an intelligence signal, comprising two vacuum tubes each having at least a cathode, grid, and anode, a source of driving voltage of carrier frequency, a resonant input circuit tuned to said carrier frequency and connecting the cathodes of said tubes in push-pull relation, means for coupling said source to said resonant input circuit, a resonant anode circuit tuned to said carrierfrequency and connecting said anodes in push-pull relation, a source of modulating intelligence signal having a frequency substantially less than said carrier frequency, means for connecting said modulating intelligence signal source between said grids and a point of substantially fixed potential, coupling means for deriving a modulated carrier signal from said resonant anode circuit, and reactive means for preventing the appearance of carrier frequency voltages :between said grids and said point of fixed potential.

6. Apparatus for producing a carrier signal modulated with an intelligence signal, comprising two vacuum tubes each having at least a cathode, grid, and anode, a source of driving voltage of carrier frequency, a resonant input circuit tuned to said carrier frequency and. connecting the cathodes of said tubes in push-pull relation, means for coupling said source to said resonant input circuit, a resonant anode circuit tuned to said carrier frequency and connecting said anodes in push-pull relation, a neutralizing condenser connected between the cathode of the first of said vacuum tubes and the anode of the second of said vacuum tubes, opposite a second neutralizing condenser connected between the cathode of the second of said vacuum tubes and the anode of the first of said vacuum tubes, a direct lowimpedance connection between said grids, a source-of modulating intelligence signal having a frequency substantially less than said carrier frequency, means for connecting said modulating intelligence signal source between said grids and a point of substantially fixed potential, coupling means for deriving a modulated carrier signal from said resonant anode circuit, and capacitive means for preventing the appearance of voltages of carrier frequency between said grids and said point of fixed potential.

7. Apparatus for producing a carrier signal modulated with an intelligence signal, comprising two space discharge devices each having a cathode, grid, and anode, an output load circuit connecting the anodes of said devices in push-pull relation, a source of driving voltage of carrier frequency, a carrier frequency input circuit connecting said cathodes in push-pull relation, means for coupling said source tosaid input circuit, a direct connection between said grids, a source of modulating intelligence signal, means for connecting said modulating intelligence sign-a1 source between said grids and a point of fixed potential, and frequency selective means associated with said voltage sources for substantially isolating said cathodes from said modulating voltage,

and for substantially isolating said grids from voltages of said carrier frequency.

8. Apparatus for producing a carrier signal modulated with an intelligence signal, comprising a space discharge device having a grid, cathode and anode, one of said two last-named electrodes being utilized as an input electrode and the other being utilized as an output electrode, a direct current source of voltage connected between said cathode and said anode, a source of driving voltage of carrier frequency, means connecting said carrier frequency source between said input electrode and a point of predetermined potential, an output load circuit, means for connecting said load circuit between said output electrode and said point of predetermined potential, a source of modulating intelligence signal, means for connecting said last-named source between said grid and said point of predetermined potential, and an electrical path having low impedance at said carrier frequency connected between said grid and said point of predetermined potential, whereby said grid is enabled to function as an electrostatic shield between said cathode and said anode.

9. Apparatus for producing -a carrier signal modulated with an intelligence signal comprising 7 a pair of space discharge devices each having a grid, cathode, and anode, a first tuned circuit connected between said cathodes, a second tuned circuit connected between said anodes, one of said tuned circuits being employed as an output circuit while the other is employed as an input circuit, a source of driving voltage of carrier frequency coupled to said input circuit, a direct current source of voltage connected between electrical centers of said tuned circuits, a direct lowimpedance connection between said grids, a source of modulating intelligence signal, means connecting said last-mentioned source between a point on said direct current source and said grids, and an electrical path having low impedance at said carrier frequency shunted across said modulating signal source, whereby each of said grids functions as an electrostatic shieldbetween anode and cathode in each of said devices.

10. A grid-modulated amplifier according to claim 9, characterized in that capacitive means are provided for preventing the capacitive transfer of energy within said space discharge devices, said means comprising a first condenser connected between the anode of the first of said devices and the cathode of the second of said devices, and asecond condenser connected between the anode of the second of said devices and the cathode of the first of said devices.

11. Apparatus for producing a carrier signal modulated with an intelligence signal, comprising a pair of space discharge devices each having a grid, cathode, and anode, a source of driving voltage of carrier frequency, a resonant input circuit tuned to said carrier frequency and connecting the cathodes of said devices in push-pull relation, means for coupling said source to said input circuit, :a resonant output circuit tuned to said carrier frequency and coupled to said anodes in push-pull relation, a direct current source of voltage connected between electrical centers of said input and output circuits, a connection between said grids having negligible impedance at both said carrier frequency and at modulating frequencies, a source of modulating intelligence signal, means connecting said last-mentioned source between a point on said direct current source and said grids, means for preventing the appearance of carrier frequency voltages between said direct current source and said grids, a first condenserconnected between the anode of the first of said devices and the cathode of the second of said devices, and a second condenser connected between the anode of the second of said devices and the cathode of the first of said devices.

12. Apparatus for producing a carrier signal modulated with an intelligence signal, comprising a pair of space discharge devices, each having a grid, cathode, and anode, an output load circuit connecting said cathodes in push-pull relation, a carrier frequency input circuit connecting said anodes in push-pull relation, a source of driving voltage of carrier frequency, means for coupling said source to said input circuit, a source of modulating intelligence signal, means for connecting said last-named source between a point of predetermined potential and said grids, a connection between said grids having negligible impedance at frequencies of said modulating signal, and means providing a low-impedance path at said carrier frequency between said point of pre-- determined potential and said grids.

13. Apparatus for producing a carrier signal modulated with an intelligence signal, comprising a pair of space discharge devices, each having a grid, cathode, and anode, an output load circuit connecting said cathodes in push-pull relation, a carrier frequency input circuit connecting said anodes in push-pull relation, a source of driving voltage of carrier frequency, means for coupling said source to said input circuit, a first neutralizing condenser connected between the anode of one of said devices and the cathode of the other of said devices, a second neutralizing condenser connected between the cathode of said one device and the anode of said other device, means for maintaining said grids substantially at ground potential with respect to' voltages of said carrier frequency, a source of modulating intelligence signal, a source of direct current voltage connected between electrical centers of said input and output circuits, and means for connecting said source of modulating signal between a point on said source of direct current and said grids, said modulating signal being applied to said grids in substantiallylike phase.

THOMAS M. GLUYAS, JR. 

